num = int(input("请输入一个数字: "))
factorial = 1

# 查看数字是负数，0 或 正数
if num < 0:
    print("抱歉，负数没有阶乘")
elif num == 0:
    print("0 的阶乘为 1")
else:
    for i in range(1, num + 1):
        factorial = factorial * i
    print("%d 的阶乘为 %d" % (num, factorial))